奈米孔洞氧化鋁在溼度感測應用上之研究

在本實驗中，我們對奈米孔洞陽極處理氧化鋁(Al2O3)表面的孔洞大小分佈及其表面沉積物改質對其在溼度感測方面上之影響做進一步的探討。陽極處理後的試片，藉由擴孔時間長短，擴孔0至360分鐘，其顯微結構有相當大的改變。擴孔時間在0、20、40、60分鐘之孔洞氧化鋁(Al2O3)，其孔洞大小範圍在38奈米至71奈米之間，將其應用於感測溼度上之研究。而表面改質則利用原子層沈積技術(Atomic layer deposition, ALD)於奈米孔洞氧化鋁(Al2O3)上沈積氧化鋅(ZnO)。除了孔洞大小及表面改質材料外，感測器表面的性質與其適用的的量測頻率範圍也對溼度感測上有重要的影響。在頻率1000赫茲到5000赫茲的範圍之間，孔洞大小在38奈米至64奈米間的試片擁有良好的感應能力。而其阻抗變化幅度在相對濕度40％到95％之間，皆有超過100倍的變化。而在表面改質材料方面，相同測試條件下，只有表面全部沈積氧化鋅(ZnO)之試片擁有和奈米孔洞氧化鋁(Al2O3)有相當的感測能力，而沈積半邊氧化鋅(ZnO)之溼度感測器則失去感測能力。我們藉由阻抗分析光譜學進一步對其感應能力的來源機制作探討，引進模擬等效電路的概念，將實際量測之電阻抗頻譜與模擬等效電路做數值擬和，得到相當吻合的結果。在等效電路中，除了使用兩個並聯的電阻電容組合外，恆定相元素(Constant phase element, CPE)的概念也將引進此等效電路中。將等效電路中的各元素在溼度變化下作統合性的比較，與其各自代表之物理和化學現象做連結，對溼度感測機制做更進一步之探討由此確認其溼度感測機制主要可分為兩大部份：其一為離子積聚效應，在低頻的量測頻率之下，吸附之水層與電極間之間的離子將會不均勻地分佈於電極和水層之間，此為其溼度感測機制之一；其二為電荷傳輸機制，高溼度下，吸附於孔洞氧化鋁表面之電荷（H+/H3O+）可傳遞經由吸附的水層上，溼度越高傳輸越易進而阻抗下降。此兩種機制共同組合成其溼度感測能力來源。

關鍵字

溼度；氧化鋁；氧化鋅；阻抗；吸附；奈米孔洞；感測器；陽極處理；原子層沉積

並列摘要

The effects of pore size and surface coating on the humidity sensing behaviors of nanoporous anodic aluminum oxide (AAO) are reported. The pore sizes of the AAOs, which range from 38 nm to 71 nm with the corresponding pore-widening time of 0 minute to 60 minutes, are studied. The chosen surface-coating material for the AAO is zinc oxide (ZnO) deposited using the atomic layer deposition (ALD) method. The humidity responses of the AAO sensors are strongly affected by their surface properties and the measuring frequency. At 1 KHz to 5 KHz, the non-coated AAO sensors with average pore diameters of 38 nm to 64 nm exhibit better sensitivities than at other measuring frequencies, and the variations in impedance are over 2 orders of magnitude at the relative humidity (RH) range of 40% to 95%. The fully-ZnO-coated AAO sensors also show good sensitivities under identical operating conditions. However, the present study shows that by halfing the ZnO-coated area, the humidity sensing ability of the AAO sensors is lost. The humidity sensing mechanisms of the AAO sensors are confirmed using electrochemical impedance spectroscopy (EIS). A proposed equivalent circuit, consisted of two RC parallel circuits and a constant phase element (CPE), provide good theoretical fittings on the Cole-Cole and Bode plots for experimental data. Two sensing mechanisms are suggested; they are: (1) the ion accumulation of ions (H+/H3O+) between the adsorption water layer and the electrodes and (2) the charge transportation (H+/H3O+) through the physisorption water layers.

並列關鍵字

Humidity ； Alumina (Al2O3) ； Zinc Oxide (ZnO) ； Anodizing ； Impedance ； sensor ； Atomic layer deposition (ALD) ； Nanoporous

參考文獻

[1] E. Traversa, "Ceramic Sensors for humidity detection: the state-of-the-art and future-developments," Sensors and Actuators B-Chemical, 23, (1995), pp. 135-156.

[2] N. Yamazoe and Y. Shimizu, “Humidity sensors: principles and applications,” Sensors and Actuators, 10, (1986), pp. 379–398.

[4] O. K. Varghese and C. A. Grimes, "Metal oxide nanoarchitectures for environmental sensing," Journal of Nanoscience and Nanotechnology, 3, (2003), pp. 277-293.

[5] Z. Chen and C. Lu, "Humidity sensors: A review of materials and mechanisms," Sensor Letters, 3, (2005), pp. 274-295.

[6] O. K. Varghese, D. W. Gong, M. Paulose, K. G. Ong, C. A. Grimes and E. C. Dickey, "Highly ordered nanoporous alumina films: Effect of pore size and uniformity on sensing performance," Journal of Materials Research, 17, (2002), pp. 1162-1171.

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奈米孔洞氧化鋁在溼度感測應用上之研究

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